In the case of a GPS system, any radio-frequency signal receiver can receive signals from satellites in orbit. The ground distance between the receiver and a visible satellite can be from 20,000 km, when one of the satellites is at zenith, to 26,000 km, when one of the satellites is at a tangential point with the surface of the Earth, i.e. on the horizon.
Currently, 30 satellites are placed in orbit at a distance close to 20,200 km above the surface of the Earth on 6 orbital planes each inclined by 55° with respect to the Equator. The time taken by a satellite to complete one revolution in orbit in order to return to the same point above the Earth is approximately 12 hours. The distribution of the satellites in orbit allows a terrestrial GPS receiver to receive the GPS signals from at least four visible satellites to determine its position, speed and the local time for example.
For civilian applications, each of these satellites transmits radio-frequency signals formed in particular of a first carrier frequency at 1.57542 GHz on which the P-code is modulated at 10.23 MHz and C/A PRN code is modulated at 1.023 MHZ peculiar to each satellite with the GPS message at 50 Hz. This GPS message contains the ephemerides and almanac data used particularly for calculating the X, Y Z position, speed and local time.
The C/A PRN code (pseudo random noise) of each satellite, which is a Gold code, is a unique pseudo random code for each satellite so that the signals transmitted by the satellites can be differentiated inside the receiver. The C/A code is a digital signal composed of 1023 chips, and which is repeated every millisecond. All of the Gold codes have the characteristic of being almost orthogonal, i.e. by correlating them with each other, the correlation result gives a value close to 0. This characteristic allows several radio-frequency signals from several satellites to be simultaneously processed independently in several channels of the same GPS receiver.
In current navigation systems that use GPS type receivers, it is often necessary to introduce an initial approximate position to facilitate the calculation of the precise position of the receiver. Consequently, this requires the receiver user to enter for example the coordinates of a place stored in proximity to his current position, which is a drawback.
In the patent application DE 196 33 477, it is described means for a radio frequency signal receiver allowing the receiver to effect a first search of a first set of satellites without any knowledge of the initial approximate position. In order to do this, search channels of the receiver are configured for the search of a certain number of satellites from at least two orbital planes sensibly perpendicular so that the signals from at least are visible satellite can be picked up by the receiver.
Since it is envisaged to mount such radio-frequency signal receivers in portable objects, such as cellular telephones or also in wristwatches, all of the precise position calculation operations must be facilitated. Moreover, the electric power consumption during these calculation operations must be greatly reduced, given that the receiver is powered in such objects by an accumulator or battery of small size.